The present invention relates to a mold clamping system of a motor-driven injection molding machine. More particularly, the present invention relates to a control device for a mold clamping system used for carrying out a clamping process with a toggle mechanism that is driven by a servo-motor.
A mold clamping system in an injection molding machine comprises a servo-motor, a ball screw, and a toggle mechanism. The servo-motor has a predetermined rated torque. The ball screw has a screw shaft and a nut to convert rotation of the servo-motor into rectilinear motion. The rotation of the servo-motor is transmitted to the screw shaft. In response to this, the screw shaft rotates and the nut moves forward, allowing the toggle mechanism to achieve clamping.
Molding of resins typically involves a plurality of processes, i.e., dispensing of resin, filling, dwelling, and cooling. Besides these processes, the mold clamping system carries out a clamping process. In the mold clamping system, a torque of the servo-motor is controlled during the clamping process in order to provide a clamping force of which a profile exhibits a predetermined pattern or patterns.
The time required for the clamping process may vary depending on the product to be molded. For example, it may take an hour for the clamping process to mold a lens. Under such circumstances, a conventional mold clamping system determines a rated torque for the servo-motor based on a maximum clamping force required for the molded product. This means that a rated power of the servo-motor used for the mold clamping system is determined so that it is capable of generating the maximum clamping force without exceeding its predetermined power rating. However, the maximum clamping force does not last for a long time in typical mold clamping systems, except for those designed for a special purpose as described above. In other words, the time during which the maximum clamping force is required accounts for a small proportion of the time for the entire clamping process.